Computer and other data handling systems have a variety of types of data storage. One common place for storing very large amounts of data is in an ordinary disc drive. The most basic parts of a disc drive are the housing, the rotatable data storage disc(s), the actuator assembly that moves a head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
To read and write data to the disc drive, the actuator assembly includes one or more arms that support the head over the disc surface. The actuator assembly is selectively positioned by a voice coil motor which pivots the actuator assembly about a pivot shaft secured to the drive housing. The disc is coupled to a motorized spindle which is also secured to the housing. During operation, the spindle provides rotational power to the disc. By controlling the voice coil motor, the actuator arms (and thus the heads) can be positioned over any radial location along the rotating disc surface.
The head is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc adjacent a data surface. Most sliders have an air-bearing surface (“ABS”) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equalize so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on portions of the storage disc referred to as tracks. Heads, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the heads are accurately positioned over one of the designated tracks on the surface of the storage disc. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write to or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is often divided between several different tracks. While most storage discs utilize multiple concentric circular tracks, other discs have tracks forming a continuous spiral on each data surface.
During manufacturing, servo information is encoded on the disc and subsequently used to accurately locate the head. The written servo information is used subsequently to locate the actuator assembly/head head at the required position on the disc surface and hold it very accurately in position during a read or write operation. The servo information is written or encoded onto the disc with a machine commonly referred to as a servo track writer (hereinafter STW). At the time the servo information is written, the disc drive is typically at the “head disc assembly” (hereinafter HDA) stage. The HDA includes most of the mechanical drive components but does not typically include all the drive electronics. During the track writing process, the STW precisely locates the head heads relative to the disc surface and writes the servo information thereon.
As demand for higher capacity drives grows, manufacturers are constantly seeking to increase drive capacity while keeping costs and cycle times low. Today, ordinary STW technology is often too limited in production volume to meet increasing market demand and stringent cost reduction targets. To address this issue, Prewritten-Servo-Patterns (PSP) are presently a promising technology. For example, in the field of MDW (Multi-Disc Writer) technology, a special disc-writing machine is applied to write servo tracks on multiple discs at a time, with the multiple-head support of the MDW machine. The written discs are then built into the drives. The main advantage of PSP technology is its ability to reduce valuable factory clean room space and cycle time through the servo track writing on several discs on a single machine, simultaneously. As such, the cost savings are estimated to be significant over ordinary non-PSP. Since the PSP machines are built with high precision specifications, the track squeeze severity on high TPI servo system can be reduced. For these reasons, PSP technology has been found to be an encouraging alternative solution over ordinary STW technology.
While promising more favorable combinations of throughput and precision, technologies like PSP have caused a host of other issues that component manufacturers have yet to address. Such devices often have novel and unaccounted-for design features such as centering offsets and/or poorly-selected region boundaries. It would be advantageous, moreover, to perform certain parametric monitorings, screens, calibrations, and similar measurements relating to observed design features on a large-scale sample of devices. Accordingly, what is needed is an apparatus and method for performing such measurements accurately and cost-effectively.